Method and Device for Remote Sensing and Control of LED Lights

ABSTRACT

A control system is disclosed for determining an actual temperature of a light emitting diode. The control system uses conductor that supply power to the light emitting diode to supply a pulse to the light emitting diode. The pulse is determined along with a reaction caused by the pulse and the information gained is used in determination of the light emitting diode die temperature which can then be used in controlling current to the light emitting diode to control the temperature of the light emitting diode.

RELATED APPLICATIONS

The present application claims priority from U.S. ProvisionalApplication Ser. No. 61/144,408, filed on Jan. 13, 2009, which isincorporated herein by reference.

BACKGROUND

An increasing number of light fixtures are utilizing light emittingdiodes (LEDs) as light sources to increased efficiency and provide alonger operational lifetime over conventional incandescent lightsources. While designers using incandescent light sources have haddecades to work out problems, LEDs are relatively new and still presentsome issues that need to be resolved before gaining wide acceptance. Onesuch issue involves the reaction of LEDs to heat. LEDs require arelatively low constant temperature in comparison to incandescent lightsources or bulbs. A typical operating temperature of an incandescentfilament is over 2,000 degrees Celsius. An LED may have a maximumoperating temperature of approximately 150 degrees Celsius, andoperation above this maximum can cause a decrease in the operationallifetime of the LED. The decrease in light output is caused at least inpart by carrier recombination processes at higher temperatures and adecrease in the effective optical bandgap of the LED at thesetemperatures. A typical operating temperature of an LED is usually belowabout 100 degrees Celsius to preserve operational lifetime whilemaintaining acceptable light output.

Multiple LEDs are typically grouped together in each light fixture toprovide the amount of light output necessary for lighting a room in ahome or building. LEDs used in light fixtures are typically considerablyhigher in light output and power consumption than the typical coloredindicator LED seen in many electronic devices. This increase in the LEDdensity and power causes an increase in heat buildup in the fixture. InLEDs, an increase in temperature causes an increase in current which,consequently, causes a further increase in temperature. If leftunchecked, the increased current caused by increased temperature cancause thermal runaway where the temperature increases to a point wherethe LED is damaged. Therefore, it is important to control the powersupplied to the LEDs to ensure that the temperature of the LEDs does notexceed the maximum safe operating temperature. Controlling the power tothe LED can generally be accomplished by controlling the current orcontrolling the voltage, although light output is directly related tocurrent.

Incandescent and fluorescent lighting fixtures in buildings are usuallysupplied by a line or mains voltage, such as 115 Volts AC at 60 Hertz inthe United States. Other single phase voltages are also used, such as277 Volts AC, and in some instances other single and multiple phasevoltages are used as well as other frequencies, such as in Britain where220 Volts at 50 Hz is common. Power to these lighting fixtures iscontrolled by a wall mounted switch for an on or off operation, and adimmer switch can be used to control brightness levels in addition toproviding a simple on and off function.

LEDs in light fixtures operate on a much lower voltage than what istypically supplied to a building. LEDs require low voltage DC so supplypower must be converted from higher voltage AC to DC constant current.Generally a single white LED will require a forward voltage of less thanapproximately 3.5 Volts. It is also important to control current to theLED since excessive current can destroy the LED and changes in currentcan lead to undesirable changes in light output.

Some conventional LED lighting systems use thermocouples or thermistorsto measure temperatures of the LEDs. These devices are placed in aposition near the LED and are connected to a temperature monitoringsystem using set of wires that are in addition to the wires powering theLED. These temperature detection devices cannot directly measure theactual temperature of the LED die itself since they necessarily have tobe spaced apart from the LED die because of optics of the LEDs and theLED conductors. In addition, the extra set of wires between thethermistor and the monitoring system can be inconvenient, especially ifthe monitoring system is a significant distance from the thermistor.Because the thermistors do not directly measure the actual temperatureof the LED die, these devices introduce some particular inaccuraciesinto the temperature measurement.

SUMMARY

The present invention overcomes the limitations of conventional LEDtemperature measurement devices by providing a method and device formeasuring an actual temperature of the LED not an approximation based ona temperature near the LED.

In one embodiment, according to the present disclosure, a method fordetermining a temperature of at least one light emitting diode (LED) ina circuit is disclosed. The circuit includes a power supply for poweringthe LED through first and second LED conductors by providing anoperating current through the LED conductors and an operating voltageacross the LED conductors. The LED is operable to generate light inresponse to receiving the operating current in a range of operatingcurrents and receiving the operating voltage in a range of operatingvoltages. An operating current and operating voltage are provided to theLED through the first and second LED conductors. A current pulse issuperimposed on the operating current to the LED, through the first andsecond LED conductors resulting in a voltage pulse that is superimposedon the operating voltage. The voltage pulse is sensed across the firstand second LED conductors resulting from the applied pulse of current todetermine a voltage magnitude of the voltage pulse. A current magnitudeof the current pulse is determined, and the operating temperature of theLED is determined based on the current magnitude of the current pulseand the voltage magnitude of the voltage pulse.

In another embodiment, a method for determining a temperature near atleast one light emitting diode (LED) in a circuit is disclosed. Thecircuit includes a power supply for powering the LED through first andsecond LED conductors to cause the LED to operate to generate light whena forward operating voltage and forward operating current is supplied tothe LED through the LED conductors. The LED exhibits a forward voltageresistance when the forward operating voltage is supplied to the LEDthrough the LED conductors and the LED exhibits a reverse bias voltageresistance when a reverse bias voltage is supplied to the LED throughthe LED conductors. The reverse bias voltage resistance is greater thanthe forward voltage resistance. A thermistor is arranged across thefirst and second LED conductors in parallel with the LED. The thermistorhas an effective resistance range in which at least two differentthermistor resistances of the thermistor correspond to at least twodifferent thermistor temperatures of the thermistor. The thermistorresistances in the effective resistance range are lower than the reversebias voltage resistance of the LED and are higher than the forwardvoltage resistance of the LED. An effective resistance range is selectedsuch that, when a forward drive current is applied to the LEDconductors, one portion of the forward drive current which flows throughthe LED is the forward operating current and another portion of theforward drive current which flows through the thermistor is a forwardthermistor current which is smaller than the forward operating current.When the reverse bias voltage is supplied to the LED conductors, areverse drive current flows through the LED conductors in an oppositedirection than the forward drive current and one portion of the reversedrive current flows as a leakage current through the LED and which doesnot cause the LED to produce light and another portion of the reversedrive current flows through the thermistor as a reverse thermistorcurrent which is larger than the leakage current. The thermistor ispositioned in a thermal pathway of the LED to receive heat produced bythe LED during operation of the LED. The temperature of the thermistoris measurable by determining the thermistor resistance using the reversethermistor current and the temperature of the thermistor is related to atemperature of the LED.

Another embodiment involves a switch assembly for electricalcommunication with at least one light emitting diode (LED) assembly tocontrol the LED assembly. The LED assembly has at least one LED with afirst LED conductor and a second LED conductor and which is powered byreceiving an operating current through the LED conductors in a range ofoperating currents and an operating voltage across the LED conductors ina range of operating voltages. The LED assembly is configured to befixedly installed remotely from the switch assembly to provide light. Atransformer is included for electrically connecting to a line powersource for receiving line power with a line voltage greater than 100Volts AC and converting the line voltage to a transformed power with atransformed voltage that is less than 50 Volts. A power controller isincluded for receiving the transformed power from the transformer andfor at least creating an operating power with the operating current inthe range of operating currents and the operating voltage in the rangeof operating voltages. The power controller is arranged to electricallyconnect to the LED assembly through the LED conductors to supply theoperating current and operating voltage to the LED assembly through theLED conductors. A temperature monitor is included for electricallyconnecting to the LED conductors. The temperature monitor includes acurrent sensor to determine a magnitude of current through the LEDconductors and a voltage sensor to determine a magnitude of voltageacross the LED conductors. The temperature monitor also includes acontroller that is electrically connected with the current and voltagesensors and is configured for calculating a temperature of the LED inthe LED assembly based at least partially on the determined current andvoltage magnitudes. A switch is also included for selectively connectingand disconnecting the operating current and operating voltage from theLED assembly.

In yet another embodiment, a control system is disclosed for determininga temperature of at least one light emitting diode (LED) in a circuitwhich includes a power supply for powering the LED through first andsecond LED conductors by providing an operating current through the LEDconductors and an operating voltage across the LED conductors. The LEDis operable to generate light in response to receiving the operatingcurrent in a range of operating currents and receiving the operatingvoltage in a range of operating voltages. The control system includes apower supply for providing the operating current and operating voltageto the LED through the first and second LED conductors. The power supplyis used in superimposing a current pulse on the operating current to theLED, through the first and second LED conductors resulting in a voltagepulse that is superimposed on the operating voltage. A voltage sensor isincluded for sensing the voltage pulse across the first and second LEDconductors resulting from the applied pulse of current to determine avoltage magnitude of the voltage pulse. A current sensor is included fordetermining a current magnitude of the current pulse. A controller isincluded for determining the operating temperature of the LED at theprovided operating current based on the current magnitude of the currentpulse and the voltage magnitude of the voltage pulse.

In another embodiment, a control system is disclosed for determining atemperature near at least one light emitting diode (LED) in a circuit.The circuit includes a power supply for powering the LED through firstand second LED conductors to cause the LED to operate to generate lightwhen a forward operating voltage and forward operating current issupplied to the LED through the LED conductors. The LED exhibits aforward voltage resistance when the forward operating voltage issupplied to the LED through the LED conductors and the LED exhibits areverse bias voltage resistance when a reverse bias voltage is suppliedto the LED through the LED conductors. The reverse bias voltageresistance is greater than the forward voltage resistance. The controlsystem includes a thermistor that is electrically connected across thefirst and second LED conductors in parallel with the LED. The thermistorhas an effective resistance range in which at least two differentthermistor resistances of the thermistor correspond to at least twodifferent thermistor temperatures of the thermistor. The thermistorresistances in the effective resistance range are lower than the reversebias voltage resistance of the LED and are higher than the forwardvoltage resistance of the LED. The effective resistance range is suchthat, when a forward drive current is applied to the LED conductors, oneportion of the forward drive current which flows through the LED is theforward operating current and another portion of the forward drivecurrent which flows through the thermistor is a forward thermistorcurrent which is smaller than the forward operating current. When thereverse bias voltage is supplied to the LED conductors, a reverse drivecurrent flows through the LED conductors in an opposite direction thanthe forward drive current and one portion of the reverse drive currentflows as a leakage current through the LED and which does not cause theLED to produce light and another portion of the reverse drive currentflows through the thermistor as a reverse thermistor current which islarger than the leakage current. The thermistor is positioning in athermal pathway of the LED to receive heat produced by the LED duringoperation of the LED. The temperature of the thermistor is measurable bydetermining the reverse thermistor current and the temperature of thethermistor is related to a temperature of the LED.

In another embodiment, a method is disclosed for electricallycommunicating with at least one light emitting diode (LED) assembly tocontrol the LED assembly. The LED assembly having at least one LED witha first LED conductor and a second LED conductor. The LED is powered byreceiving an operating current through the LED conductors in a range ofoperating currents and an operating voltage across the LED conductors ina range of operating voltages. The LED assembly is configured to befixedly installed to provide light. The method includes transforming aline power source with a line voltage greater than 100 Volts AC andconverting the line voltage to a transformed power with a transformedvoltage that is less than 50 Volts. The transformed power is receivedand an operating power is created with the operating current in therange of operating currents and the operating voltage in the range ofoperating voltages. The operating current and operating voltage isselectively supplied to the LED assembly through the LED conductors tocontrol light output of the LED assembly. A temperature of the LED isdetermined through the LED conductors at least partially by determininga magnitude of current through the LED conductors and determining amagnitude of voltage across the LED conductors.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood by reference to the followingdetailed description taken in conjunction with the drawings, in which:

FIG. 1 is a block diagram of a control system for determining atemperature of a light emitting diode.

FIG. 2 is an equivalent circuit diagram of a light emitting diode usedfor determining the temperature of the light emitting diode.

FIG. 3 is a graph of experimental and theoretical results fortemperatures determined.

FIG. 4 is a block diagram of a circuit for determining an ambienttemperature.

FIG. 5 a is a block diagram of a circuit for determining an ambienttemperature using conductors for powering the light emitting diode.

FIG. 5 b is another block diagram of the circuit for determining anambient temperature using conductors for powering the light emittingdiode.

FIG. 6 is a diagrammatic illustration, in elevation, of a control systemhaving a switch for controlling a light emitting diode mounted in alight fixture in a room.

FIG. 7 is a block diagram of a control system having an interface fortransferring data to a supervisory system.

FIG. 8 is a block diagram of a control system for determining atemperature of more than one light emitting diode.

FIG. 9 is another block diagram of a control system for determining atemperature of more than one light emitting diode.

FIG. 10 is a flow diagram illustrating a method for determining atemperature of at least one light emitting diode.

FIG. 11 is a flow diagram illustrating a method for determining atemperature near at least one light emitting diode in a circuit.

FIG. 12 is a flow diagram illustrating a method for electricallycommunicating with at least one LED assembly to control the LEDassembly.

DETAILED DESCRIPTION

While this invention is susceptible to embodiment in many differentforms, there are shown in the drawings, and will be described herein indetail, specific embodiments thereof with the understanding that thepresent disclosure is to be considered as an exemplification of theprinciples of the invention and is not to be limited to the specificembodiments described. Descriptive terminology such as, for example,uppermost/lowermost, right/left, front/rear and the like has beenadopted for purposes of enhancing the reader's understanding, withrespect to the various views provided in the figures, and is in no wayintended as been limiting.

Referring to the drawings, wherein like components may be indicated bylike reference numbers throughout the various figures, FIG. 1illustrates one embodiment of a light emitting diode (LED) controlsystem, indicated by the reference number 100 within a dashed line.Control system 100 is electrically connected to an LED 102 using a firstLED conductor 104 and a second LED conductor 106. LED 102 is operable toproduce light, represented by arrows 108, when the LED receives anoperating current 110 through the LED conductors in a range of operatingcurrents and receives an operating voltage 112 in a range of operatingvoltages across the LED conductors. In the present embodiment, controlsystem 100 includes a power supply 114 which is connected to a linepower source 116 through line conductors 118. The utility power sourceprovides an AC line voltage at a typical line voltage, such as 110 VoltsRMS, to power supply 114. Power supply 114 converts the utility power tothe operating voltage in the range of operating voltages and applies theoperating voltage to the LED conductors. Power supply 114 also convertsthe utility power to operating current 110 in the range of operatingcurrents and applies the operating current to the LED conductors.

A separate transformer (not shown in FIG. 1) can be used fortransforming the line power from the line voltage source to atransformed power. In this instance, the power supply receives thetransformed power from the transformer and produces the operatingcurrent and operating voltage as discussed.

Power supply 114 can be controlled by a controller 120 through a controlline 122. Controller 120 provides a current control signal 124 whichcontrols the amount or magnitude of the operating current applied to theLED conductors. The amount of light produced by the LED is directlyrelated to the amount of operating current that the LED receives.Therefore, by controlling the operating current, the controller cancontrol the amount of light produced by the LED. Controller 120 can beconnected to a current measurement analog to digital (A/D) converter 126or other current sensor which detects the magnitude of the operatingcurrent and produces a current sensed signal 128 that is supplied to thecontroller through a current sensed signal line 130. Using the currentsensed signal, the controller can determine the present level of theoperating current and can change current control signal 124 to adjustthe magnitude of the operating current. As an alternative or in additionto sensing the current with the current sensor, the current can bedetermined by producing the current at a known magnitude.

A voltage measurement A/D converter 132 or other voltage sensor isconnected between the first and second LED conductors using voltagesensor conductors 134 and 136. Converter 132 detects the voltage acrossthe LED conductors and produces a voltage sensed signal 138 on a voltagesensed signal line 140. The voltage sensed signal line is connected tocontroller 120, which receives the voltage sensed signal 138 and candetermine the operating voltage that is supplied to LED 102. It shouldbe noted that while LED 102 is presently discussed as a single LED, manyof the concepts and embodiments are applicable to multiple LED's aswell. Specific examples of multiple LED systems will also be discussedbelow.

Controller 120 can include a processor 142, a clock 144 and a memory 146along with software, not specifically shown, which enables thecontroller to determine the operating current and the operating voltagebased on the current sensed signal 128 and the voltage sensed signal138, respectively. The software can be configured to operate thecontroller as required in view of the overall disclosure. Controller 120can also store values of the operating current and voltage in the memoryalong with the times at which the stored values occurred, among otherthings.

Current flowing through the LED causes the LED to produce heat as wellas light. The LED is operable at an operating temperature which is at asafe level if it remains below a maximum temperature. If the temperatureexceeds the maximum temperature then the LED can be subject to thermaldamage which can reduce the lifetime of the LED or cause rapid failureof the LED. In some instances, the heat causes an internal resistance ofthe LED to decrease which, in turn, increases the amount of current thatflows through the LED which increases the heat produced. Left unchecked,the LED enters a condition of thermal runaway where the heat caused bythe increased current which is caused by the heat eventually causes thetemperature of the LED to exceed the maximum temperature and the LEDfails.

In the present embodiment, control system 100 can determine thetemperature of the LED based on electrical measurements through the twoLED conductors. This allows the control system to set the operatingcurrent to prevent the LED from over heating as well as allowing for thedetermination of the operating lifetime of the LED, among other things.

Turning now to FIG. 2, a diode equivalent circuit 150 is shown connectedto first LED conductor 104 and second LED conductor 106. In general, LED102 can be represented by the diode equivalent circuit which includes adiode junction 152, a series resistance 154 and a shunt resistance 156.Using the diode equivalent circuit, operating current 104 can bedetermined by the following ideal diode equation:

$\begin{matrix}{I = {I_{o}{{\exp \left\lbrack \frac{V - E_{g}}{AkT} \right\rbrack}.}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Where I is the operating current flowing through the LED, I_(o) is aconstant depending on the LED properties, V is a voltage applied acrossthe diode junction of the LED, E_(g) is a value that is closely relatedto the optical band gap of the semiconductor at the diode junctionreferred to as the “effective” optical band gap, A is a constant knownas the diode factor which is usually a value between 1 and 2, k isBoltzmann' s constant and T is the temperature of the semiconductordiode junction in degrees Kelvin.

At relatively lower voltages, below about 1.5 to 2 volts, shuntresistance 156 of the equivalent circuit appears to dominate and thecurrent-voltage-temperature behavior deviates from what is predicted byEquation 1. However, at these lower voltages the LED remains cool andproduces little useful light. At relatively higher voltages, above about2.5 volts, series resistance 154 appears to dominate and thecurrent-voltage-temperature behavior deviates from Equation 1. Thesehigher voltages are near the limit of LED operation.

The effective shunt resistance is a result of surface and junctionimperfections while the series resistance results from sheet resistanceof the semiconductor doped layers, contact resistance and the wires. Inreverse bias, the effective shunt resistance is extremely high as thejunction depletion width increases. This insulating layer allows foressentially no current flow through the reverse biased LED. In order toavoid the accumulation of destructive levels of electrostatic charge, aZener diode (not shown) is usually placed across the diode to drain offcurrent at voltages above about 5 volts.

Equation 1 describes the current, voltage and temperature operation ofthe LED to an acceptable level of accuracy within a range of operatingvoltages where the operating voltage is above where the shunt resistancedominates and below where the series resistance dominates. In oneembodiment, this range is from about 1.5 V to about 2.5 V, however thisrange may be larger or smaller depending on characteristics of the LED.By knowing the values, other than T in Equation 1, the temperature ofthe actual diode die itself can be determined.

One of the values needed to determine the temperature is the effectiveoptical band gap value, E_(g). The effective optical band gap is nearlythe same for all white LED's since most LED's use blue light to producethe white light, even when different semiconductor materials are used.In many white LED's, the blue or UV light is used to excite phosphor toproduce white light in the white LED's. Applicant has empiricallydemonstrated with several commercial LED's that the effective opticalband gap is 3.2 eV. The diode factor A is taken to be 2, which isusually a good assumption for LED's for diodes where junctionrecombination dominates. The effective band gap can also be determinedfor the LED by solving Equation 1 for E_(g) if all of the othervariables in Equation 1 are known.

Another value that can be determined is the series resistance 154. Giventhe limitations of series and shunt resistances, it can be important tomeasure the current voltage relationship in the proper range of values.By determining the series resistance the temperature can be determinedbelow where the series resistance dominates and where an acceptablelevel of accuracy can be obtained. At high current the voltage appliedto the LED drops across the diode as well as the series resistance. Theactual diode voltage can be devolved from the total operating voltage112 from the equivalent circuit shown in FIG. 2. The operating voltage,V at a measured current I is divided across the two circuit elements Rsand the diode 152 as follows in Equation 2 where Rs is the seriesresistance and Vd is the voltage across the diode.

V=IRs+Vd  Equation 2.

Because the series resistance Rs is a constant and does not vary, theexponential dependence of the diodes I-V relationship can be separatedfrom the linear relationship of the series resistance by performingmeasurements at several different currents and voltages and solving forthe diode's variables. Equation 2 can be solved for the diode voltageVd=V−IRs which can then be substituted into Equation 1 to give thefollowing Equation 3 for determining the series resistance.

$\begin{matrix}{I = {I_{o}{{\exp \left\lbrack \frac{\left( {V - {IRs}} \right) - E_{g}}{AkT} \right\rbrack}.}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

A comparison of experimental data with results obtained using Equation 1is shown in a graph 160 in FIG. 3. Graph 160 shows a plot of a log ofoperating current 110 plotted against operating voltage 112 at threedifferent experimental temperatures; 260 degrees Celsius, 295 degreesCelsius and 383 degrees Celsius (hereinafter C). A value of 5×10² Ampswas used in Equation 1 for I_(o) to make the calculated data fit theexperimental data. The experimental data was obtained by immersing theLED in a temperature controlled mineral oil bath while the data wastaken. A thermocouple was welded to a metal slug of the LED to measurethe temperature of the LED and oil bath and the operating current andoperating voltage were measured.

The experimental results for the log current vs. voltage points at 260 Care shown by small circular dots, some of which are indicated by thereference number 162, the temperature curve of theoretical resultsobtained using Equation 1 for the temperature of 260 C are representedby dashed line 164. Data points for the experimental results of thecurrent vs. voltage at 295 C are shown as X's, some of which areindicated by the reference number 166. Solid line 168 is a temperaturecurve that shows the theoretical results obtained using Equation 1 withthe 295 C temperature. The experimental results for current vs. voltageat 383 C are shown by circular dots, some of which are indicated by thereference number 170. A temperature curve of the theoretical resultsobtained using Equation 1 at 383 C are shown by dashed line 172. Key 174also shows which information is experimental and which was obtainedusing Equation 1 for subsequently generating the three linear plots.

As shown in graph 160, at relatively higher currents and voltages theexperimental current vs. voltage points deviate from those predicted byEquation 1. While not intending to be bound by theory, this may be atleast partially due to the series resistance of the LED and mayindicative of wasted power in the form of heat. It is likely that lowtemperature measurements are affected by self heating of the LED's. Areasonably accurate determination of temperature can be obtained by thetheoretical results by using current and voltage that correspond toareas of the temperature curve where there is sufficient agreementbetween experimental and theoretical results.

As seen in graph 160, as the temperature is increased, the temperaturecurve moves to the left and the slope of the curve decreases. Thereforeby determining a point on the graph of current and voltage of an LED,the temperature of the LED can be determined based on where the pointfalls on the graph. Also, by determining more than one point based onmore than one current and voltage, the slope of the temperature curvecan be determined which can then establish the temperature for themultiple points. Further, by using Equation 1, a given current andvoltage can be used to determine a single temperature of the LED at agiven time.

Control system 100 (FIG. 1) can determine the temperature of LED 102through the two wire connection to the LED using the first and secondLED conductors 104 and 106. Operating current 110 can be provided bypower supply 114 at a known amplitude. Operating voltage 112 can bedetermined by voltage measurement A/D converter 132. Given thisinformation along with Equation 1 as discussed above, controller 120 candetermine the operating temperature of LED 102 based on current flow andvoltage across the LED conductors. This allows the temperature of theLED to be determined from a location that is remote from the LED withoutthe need for additional wires to connect to the LED. This also providesan accurate determination of the actual temperature of the LED dieitself, not the temperature of the atmosphere around the LED as providedby a thermal detection device that is located in the thermal pathway ofthe LED.

In one embodiment, controller 120 controls power supply 114 to produce apulse 111 of current that is superimposed on operating current 110through the LED conductors. This current pulse can be produced at aknown magnitude or the current can be accurately measured with currentmeasurement A/D converter 126. Current pulse 111 is shown in the presentexample as a negative pulse which lowers the operating current whilestill keeping the operating current positive, but other pulse shapes canalso be used. The current pulse causes voltage 112 to react with acorresponding voltage pulse 113 which can be measured using the voltagemeasurement A/D converter 132. Voltage pulse 113 is a temporaryreduction in operating voltage that still maintains the forward biasacross the LED. Controller 120 then uses the amplitudes of the currentand voltage pulses to determine the temperature of the LED usingEquation 1. A voltage pulse can be used in place of the current pulse.In this instance, the voltage pulse would be applied to the LEDconductors at an amplitude that is either known or sensed and theresulting current pulse can be measured using the current measurementA/D converter. It should be understood that measurements of current orvoltage pulses can be accomplished in a number of different ways in viewof the recognitions that have been brought to light herein. In oneembodiment, an average temperature of groups of LEDs that are arrangedin series or parallel can be determined based on one or more currentpulses through the LEDs.

The current pulse can be an increase or a decrease in the operatingcurrent, and the current pulse can also be in the shape of a ramp,triangle wave or other shape that provides more than one current. In thecase where the current pulse includes a shape such as the ramp, thecurrent pulse will provide more than one different current amplitudewhich, in response, will cause the voltage to exhibit more than onedifferent voltage amplitude. These multiple corresponding currents andvoltages can then be used to determine the temperature either based onpoints on a graph, such as graph 160, or based on a slope of atemperature curve. In one example, the current pulse can be used to putthe corresponding sensed voltage in a voltage range, described above,between where the series resistance of the LED and the shunt resistanceof the LED dominate. Multiple different pulses can also be used and thepulses can be produced at regular intervals, or based on the temperaturedetermined or on other parameters. In some instances, power supplies canprovide anomalies such as ripples in the current which can be used asthe current pulse. Switching type power supplies are one example ofthese types of devices.

In one embodiment, the current pulse can be sufficiently short induration such that any change in light output by the LED caused by thepulse is not perceivable by humans. This avoids any perceived flickeringof the light level that would not be desirable in a lighting system thatis used at least partially for illumination for human perception. Highpersistence phosphors can be used so that a longer pulse duration can beused. The longer pulse can improve the accuracy of the temperaturedetermination by allowing for the use of a more accurate A/D converterwhich uses a longer sampling time and can average out random noises andother interference.

By determining the temperature of the LED, control system 100 cancontrol the operating current to the LED so that the LED temperature ismaintained at a safe operating temperature below which heat damage tothe LED can occur. Controller 120 can be programmed with the maximumsafe operating temperature of the LED and can compare the determinedtemperature with the safe operating temperature. The controller canraise or lower the operating current until the LED operates at a desiredoperating temperature. The controller can also provide other controlfunctions.

The control system can also record the determined temperatures to a filein the memory along with the time of the temperature. In this way, thecontrol system can keep a running tally of the operating temperature ofthe LED and time of operation of the LED to project the lifetime of theLED. The memory can be non-volatile memory so that the system canremember the temperature of the LED in the event of a brief powerfailure. When power is restored, this allows the control system toresume operation of the LED by setting the operating current based atleast partially on the stored operating temperature. Operating currentand/or operating voltage or other parameters can also be stored intomemory for tracking other information regarding the LED. For instance,by tracking operating current, operating voltage and time, the controlsystem can monitor power consumption of the LED. Overall operating timeof the LED can be tracked by monitoring the time that operating currentand/or operating voltage are applied to the LED.

Yet another embodiment is illustrated in FIG. 4, where a control system180 includes a connection with a thermistor 182. Thermistor 182 iselectrically connected to controller 120 with thermistor conductors 184so that the controller can determine a thermistor temperature at thelocation of the thermistor by determining a resistance of thethermistor. In the embodiment shown in FIG. 4, thermistor 182 is locatedremote from LED 102 but is in the same thermal environment as the LED.By being in the same environment, thermistor 182 and LED 102 areessentially at the same temperature prior to operation of the LED atstartup or after the LED has had sufficient time to cool to the ambienttemperature after operation. In this way, thermistor 182 can be used todetermine a temperature of the LED. This operation could be conductedafter the LED and control system are installed where they are to beused, or could be conducted during a manufacturing process prior toinstallation. Thermistor 182 can also be mounted near the LED and/orwithin the same enclosure as LED 102, such as within a light fixture. Inthis case, the thermistor conductors would reach from the controller tothe location of the thermistor near the LED. Also, this allows thecontrol system to be in a different thermal environment from the LED andthermistor.

In another embodiment, shown in FIGS. 5 a and 5 b, a thermistor 186 iselectrically connected in parallel with LED 102 and is positioned in athermal pathway to receive heat from the LED. Thermistor 186 is chosento have an effective resistance range that is lower than an effectiveresistance of the LED when reverse biased. At a reverse bias voltage, upto about −5 Volts, the reverse bias resistance of the LED is extremelyhigh. The thermistor can also be chosen to have a forward voltage biasresistance that is much higher than an effective forward voltageresistance of the LED. As shown in FIG. 5 a, a control system 188 canapply a current pulse 189 to the LED conductors such that a reverse biasvoltage pulse 191 is created and a reverse bias voltage 199 is seenacross the LED. The current pulse 189 causes a reverse drive current 193in the LED conductors. One portion of the reverse drive current flowsthrough the LED as a leakage current 195 which does not cause the LED toproduce light. Another portion of the reverse drive current flowsthrough the thermistor as a reverse thermistor current 197.

By selecting the resistance range of the thermistor to be small enoughin comparison to the reverse bias resistance of the LED, the leakagecurrent flow through the LED is insignificant compared with the reversethermistor current flow through the thermistor and therefore the leakagecurrent can be ignored while still gaining a reasonably accuratetemperature measurement from the thermistor. When determined at startupor when the LED is at ambient temperature, the temperature of thethermistor is the same as the LED temperature. Connecting the thermistorin parallel with the LED allows the thermistor to be positioned with theLED and away from the control system while maintaining the advantage ofonly using the two LED conductors for powering the LED and fortemperature determination.

Operation of the LED to produce light is shown in FIG. 5 b where aforward drive current 201 is applied to the LED conductors. Forwarddrive current 201 includes one portion which flows through the LED andis referred to as the forward operating current 203 and another portionthat flows through the thermistor which is referred to as a forwardthermistor current 205. The forward drive current produces a forwardoperating voltage 207 across the LED.

By determining an ambient temperature of the LED, applying a currentpulse to the LED, determining a magnitude of the current pulse and theresulting magnitude of voltage pulse, the controller has threevariables; current, voltage and temperature, that can be used inEquation 1. Using the values determined for these variables, andsupplying known or estimated values for other parameters, the controllercan solve Equation 1 for any one of the remaining parameters. Forinstance, knowing the operating current, operating voltage, temperature,I_(o) and the diode factor A, the controller can calculate the effectiveband gap E_(g). By knowing the other variables, the diode factor can becalculated. The thermistor temperature can also be used in a calibrationprocedure to increase the accuracy in later determining the temperatureusing the operating current and operating voltage. The thermistor can beincluded in the diode package along with the LED die and in some cases azener diode.

As can be understood by the present disclosure, the control system ofthe several embodiments disclosed can be located remotely from the LEDor within the same enclosure as the LED, such as within a light fixture.One or more components of the control system can also be arrange on oneor more integrated circuits which can be included in a single LEDpackage along with the LED die.

One embodiment in which the control system is located remotely from theLEDs is shown in FIG. 6. In this embodiment, the control system isincluded in a switch assembly 190 that is installed at a fixed locationin a wall 192. Switch assembly 190 includes a switch 191 for controllingthe application of power to LED 102 through LED conductors 104 and 106.LED 102 can be installed in a lighting fixture 194 that can be mountedin a fixed position in a ceiling 196 within a room 198 with wall 192 andswitch assembly 190. The switch can be a line voltage switch in whichcase the line voltage is controlled by the switch before it is passed tothe control system. In the embodiment shown in FIG. 6, the line voltageis connected to a control system 200 using line conductors 118. Switch191 is connected using switch conductors 204 to controller 120 withinthe control system to control power to the LEDs through the controlsystem. Switch assembly 190 can have an on/off function and/or dimmingcapabilities through control by the controller. A display 206 can beincluded and connected to the controller with a display conductor 202 toindicate the status of the switch and/or the LED to a user. The displaycan be one or more colored indicators or can be a screen type display.

Switch assembly 190 can be configured to fit within and connect to aconventional single-gang electrical box 210 such as those typically usedfor mounting a conventional single-pole line voltage switch in a wall.

Another embodiment of the control system is shown in FIG. 7. Controlsystem 220 includes an interface 222 for transferring data gathered bythe control system to a supervisor system 224. Data can be transferredfrom controller 120 to interface 222 over an interface conductor 223.The supervisor system can be a system that is used for building controland/or monitoring and can receive data gathered by multiple controlsystems controlling multiple LEDs at different locations. Informationregarding power usage, temperature, operable lifetime of the LED andother useful information based on time, temperature, current and/orvoltage can be transmitted between control system 220 and supervisorsystem 224. Control system 220 and/or supervisor system 224 can includedisplays for notifying users of the status of the LED. The supervisorsystem can provide control instructions to the control system to causeit to control light output. The interface can connect to the supervisorsystem using a cable 226 such as an Ethernet cable, over the lineconductors 118 or can use wireless communications such as a ZigBee™ orother type of wired or wireless communication to a building informationsystem.

Another embodiment is shown in FIG. 8 in which a control system 250 isarranged for monitor and control of multiple LEDs. Control system 250includes a power supply 251 with a transformer 252 that is connected toreceive line power from line power source 116 through line powerconductors 118. Transformer 252 transforms the line voltage, which is115 Volts AC in the present case, to 12 Volts AC. Line voltage istypically over 100V AC, for the present embodiment. Power supply 251also includes a current controller 256. The transformer is connected tocurrent controller 256 using a power supply conductor 258 to supply the12 Volt power to the current controller. Power supply 251 is configuredto use the 12 Volts AC to supply an operating current and operatingvoltage to multiple LEDs. For illustrative purposes in the presentembodiment, the current controller is connected to two different LEDs,LED 260 and LED 262. LEDs 260 and 262 can be separate LEDs in onefixture, can be separate LEDs in separate fixtures or each LED 260 and262 can each represent multiple LEDs that are connected together and/orwith other LEDs within a similar thermal environment in a series,parallel or series-parallel circuit arrangement. LED 260 is connected tocurrent controller 256 using LED conductors 264 and LED 262 is connectedto current controller 256 using LED conductors 266. Current controller256 powers LEDs 260 and 262 through the respective LED conductors.

Control system 250 also includes a temperature monitor 270 that isconnected to the current controller using a control line 272.Temperature monitor 270 includes a processor as well as current andvoltage A/D converters that are not specifically shown in this example.Temperature monitor 270 sends control signals over control line 272 tothe current controller to set the operating current to each of the LEDs.Based on the known operating characteristics and limitations of theLEDs, a processor of the temperature monitor can regulate the operatingcurrent to the LEDs via controlling the DC current or pulsed DC current.That control may make use of user preferences to maintain constant lightoutput and/or to maintain long life. Temperature monitor 270 can alsocontrol the current controller to produce current pulses for measurementpurposes over each of the LED conductors.

A multiplexer 274 is connected to LED conductors 264 and 266 usingmultiplexer conductor lines 276 and 278, respectively. Multiplexer 274is connected to the temperature monitor using a control line 280 and asignal line 282. Temperature monitor 270 controls the multiplexerthrough the control line to selectively receive signals from one or theother of LED conductors 264 or 266 through multiplexer conductor lines276 or 278. The multiplexer then passes the selected signals to thetemperature monitor through signal line 282. The temperature monitorthen determines the current and voltage on the selected LED conductorand calculates the corresponding temperature of the LED connected to theselected LED conductor. Temperature monitor 270 can then control thecurrent controller to adjust the operating current of the selected LEDbased on the temperature. This process can then be repeated for the LEDthat was not previously selected. In this way, each of the LEDs in thesystem can be monitored for temperature, current, voltage and powerusage so long as they have a separate electrical connection to controlsystem 250.

Control system 250 can be included in a switch assembly with a switch,as previously discussed, and the switch assembly can be arranged forinstallation in a wall of a room to control LEDs supplying light to theroom. Control system 250 can also include an interface for communicatingwith a supervisory system as previously discussed. Using control system250 with a supervisory system allows the supervisory system to monitorand/or control multiple LEDs on an individual basis.

Another multiple LED arrangement is shown in FIG. 9 wherein a controlsystem 300 is connected to two LEDs 302 and 304 that are electricallyconnected in series. The number of LEDs shown in FIG. 9 is exemplary ofa system with multiple LEDs. The LEDs connect to the control systemusing LED conductors 310 and 312. LEDs 302 and 304 each include anintegral heat sink 306 and 308, respectively, that are electricallyconnected to the LED die. The heat sinks are electrically connected toan LED conductor 312 using conductors 314 and 316. LED 302 is connectedto LED conductor 310 using a first power terminal and an LED conductor320 connects a second power terminal of LED 302 to a first powerterminal of LED 304. A second power terminal of LED 304 is connected toan earth ground using a ground conductor 322.

Control system 300 includes a power supply 330 having a transformer 332and a power controller 334. Transformer 332 receives line power frompower source 116 through line power conductors 118 and transforms theline power from a higher voltage to a lower voltage which is transferredto the power controller through a power supply conductor 338.Transformer 332 can be electronic or electro-magnetic. Control system300 also includes a temperature monitor 340 which can have amicroprocessor controller. Temperature monitor 340 is connected to thepower controller using a control line 342 to pass control signalsbetween the temperature monitor and the power controller. Powercontroller 334 supplies power to the LEDs at an operating voltage and atan operating current controlled by the temperature monitor. A currentA/D converter 344 is connected to the temperature monitor using acontrol line 346 and a signal line 348. In this arrangement, each LEDeffectively has three terminals. Pulses from power controller 334 can besensed at each LED using current A/D converter 344 through theconductors 314 and 316 as the pulse passes through each LED. Controlsystem 300 can also include a control switch and can be arranged to fitwithin the volume envelope of a typical single-gang junction box.

Each of the LEDs or groups of LEDs can also include an electronic modulewith electronics that respond to an analog or digital signal command.The signal commands can originate from a controller in a wall switch, orother location. Each LED module can respond to such commandsindividually back to the controller via the conductors 314 or 316 byproducing a pulse which the controller can detect through current A/D344. The electronic module can also be arranged to periodically producea pulse that is unique for each LED or group of LEDs. The electronicmodule can also be configured to divert all or a portion of the currentflowing through conductor 310 to the conductors 314 or 316 and on to thecurrent A/D. The controller can record the current-voltagecharacteristics and determine a temperature for the LED connected to themodule. The controller can then send a signal to have the module adjustcurrent in the LED as required.

Each of the LEDs or groups of LEDs can also have a passive or activefilter tuned to a different frequency. In this instance, the filter canbe used to address the LED individually. The pulse can include afrequency component which allows the pulse to be received by a selectedindividual or group of LEDs to allow the temperature of the selected LEDto be determined.

The control system described herein can be used as a ballast for LEDlighting fixtures and much of the control system can be made in a singleIC. The control system allows the determination of the actualtemperature of an LED, not an estimated temperature based on atemperature near the LED. The control system can operate using only thetwo wires normally connected to power the LED. By using centralizedcontrol and monitor of the LED temperatures cost for LED fixtures can bereduced over systems in which each fixture includes temperaturemonitoring and control.

A method 500 is shown in FIG. 10 for determining a temperature of atleast one LED. Method 500 begins at a start 502 and then proceeds to astep 504 where an operating current and operating voltage are providedto the LED through first and second LED conductors. Following step 504,method 500 proceeds to step 506 where a current pulse is superimposed onthe operating current to the LED through the first and second LEDconductors. This results in a voltage pulse that is superimposed on theoperating voltage. After step 506, method 500 proceeds to step 508 wherethe voltage pulse is sensed across the first and second LED conductorsto determine a magnitude of the voltage pulse. Method 500 then proceedsto step 510 where a current magnitude of the current pulse isdetermined. Method 500 then proceeds to step 512 where an operatingtemperature of the LED is determined based on the current magnitude ofthe current pulse and the voltage magnitude of the voltage pulse. Method500 then ends at step 514.

A method 520 for determining a temperature near at least one LED in acircuit is shown in FIG. 11. Method 520 begins at a start step 522 andthen proceeds to a step 524 where a thermistor is arranged across thefirst and second LED conductors in parallel with the LED. The thermistorcan have an effective resistance range in which at least two differentthermistor resistances of the thermistor correspond to at least twodifferent thermistor temperatures of the thermistor. The thermistorresistances in the effective resistance range are lower than the reversebias voltage resistance of the LED and are higher than the forwardvoltage resistance of the LED. Following step 524, method 520 proceedsto step 526 where the effective resistance range is selected such that,when a forward drive current is applied to the LED conductors, oneportion of the forward drive current which flows through the LED is theforward operating current and another portion of the forward drivecurrent which flows through the thermistor is a forward thermistorcurrent which is smaller than the forward operating current.

When the reverse bias voltage is supplied to the LED conductors, areverse drive current flows through the LED conductors in an oppositedirection than the forward drive current and one portion of the reversedrive current flows as a leakage current through the LED and which doesnot cause the LED to produce light and another portion of the reversedrive current flows through the thermistor as a reverse thermistorcurrent which is larger than the leakage current. Following step 526,method 520 proceeds to step 528 where the thermistor is positioned in athermal pathway of the LED to receive heat produced by the LED duringoperation of the LED. The temperature of the thermistor is measurable bydetermining the thermistor resistance using the reverse thermistorcurrent and the temperature of the thermistor is related to atemperature of the LED. After step 528, method 520 proceeds to step 530where the method ends.

A method 550 for electrically communicating with at least one LEDassembly to control the LED assembly is shown in FIG. 12. Method 550begins at a start step 552 and then proceeds to a step 554 where a linepower source is transformed. The line power source having a line voltagegreater than 100 Volts AC and the line voltage is converted to atransformed power with a transformed voltage that is less than 50 Volts.Following step 554, method 550 proceeds to step 556 where thetransformed power is received and an operating power with at least theoperating current in the range of operating currents and the operatingvoltage in the range of operating voltages is created. After step 556,is a step 558 where the operating current and operating voltage areselectively supplied to the LED assembly through the LED conductors tocontrol light output of the LED assembly. Following step 558 is a step560 where a temperature of the LED is determined through the LEDconductors at least partially by determining a magnitude of currentthrough the LED conductors and determining a magnitude of voltage acrossthe LED conductors. After step 560, method 550 ends at step 562.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

1. A method for determining a temperature of at least one light emittingdiode (LED) in a circuit which includes a power supply for powering theLED through first and second LED conductors by providing an operatingcurrent through the LED conductors and an operating voltage across theLED conductors, where the LED is operable to generate light in responseto receiving the operating current in a range of operating currents andreceiving the operating voltage in a range of operating voltages, themethod comprising: providing the operating current and operating voltageto the LED through the first and second LED conductors; superimposing atleast one current pulse on the operating current to the LED, through thefirst and second LED conductors resulting in a voltage pulse that issuperimposed on the operating voltage; sensing the voltage pulse acrossthe first and second LED conductors resulting from the applied pulse ofcurrent to determine a voltage magnitude of the voltage pulse;determining a current magnitude of the current pulse; and determining anoperating temperature of the LED at the provided operating current basedon the current magnitude of the current pulse and the voltage magnitudeof the voltage pulse.
 2. The method according to claim 1, furthercomprising: before providing the operating current, providing an initialoperating current and initial operating voltage at a known initialtemperature of the LED; using measured values of the initial operatingcurrent and initial operating voltage and initial temperature todetermine at least one LED parameter of the LED and wherein determiningthe operating temperature includes subsequently using the LED parameter.3. The method according to claim 2 wherein the initial operating currentand initial operating voltage are provided at a time when the LED hasreached a resting temperature that is essentially the same as an ambienttemperature around the LED.
 4. The method according to claim 3 whereindetermining the operating temperature includes using the LED parameterin an ideal diode equation.
 5. The method according to claim 4 whereinthe LED parameter is related to an effective band gap of the LED.
 6. Themethod according to claim 4 wherein the LED parameter is used todetermine a current to voltage curve for at least one temperature. 7.The method according to claim 1 wherein the superimposed current pulseis a first current pulse and the voltage pulse is a first voltage pulse,and further comprising: superimposing at least a second, differentcurrent pulse on the operating current which results in a second,different voltage pulse that is superimposed on the operating voltage;deriving an operating temperature curve from a logarithm of a magnitudeof the current pulses against a magnitude of the voltage pulses;determining a slope of the operating temperature curve; and determiningthe operating temperature of the LED based on the determined slope ofthe operating temperature curve.
 8. The method according to claim 7wherein the operating temperature is determined by comparing the slopeof the operating temperature curve to predetermined slope values thatcorrespond to different LED temperatures.
 9. The method according toclaim 1 wherein the current pulse is superimposed on the operatingcurrent at a predetermined magnitude such that the determined currentmagnitude is the predetermined magnitude.
 10. The method according toclaim 1 wherein superimposing includes selecting a power supply thatexhibits a ripple current and using the ripple current as the currentpulse.
 11. The method according to claim 1 wherein the circuit includingthe LED and the power supply is part of a device to at least provideillumination to assist visual perception of humans, and where thesuperimposition of the current pulse on the operating current produces atemporary change in the light level from the LED and the temporarychange in the light level is less than an interval having a durationthat is imperceptible to humans.
 12. The method according to claim 1,wherein the LED is operable at a safe operating temperature that is lessthan a maximum temperature and is subject to thermal damage at a damagetemperature that exceeds the maximum temperature, the method furthercomprising: comparing the operating temperature with the maximumtemperature; and reducing the operating current responsive to theoperating temperature exceeding the maximum temperature.
 13. The methodaccording to claim 1 wherein the superimposed current pulse lowers themagnitude of the operating current to a value that is less than anothervalue of the operating current that would otherwise be present withoutthe superimposed current pulse.
 14. The method according to claim 1further comprising: arranging a thermistor across the first and secondLED conductors in parallel with the LED such that the current pulseflows through a parallel combination of the LED and the thermistor, andselecting the thermistor to have an effective resistance range that islower than a reverse bias resistance of the LED and that is higher thana forward voltage resistance of the LED; and wherein the superimpositionof the current pulse causes the current to the LED to temporarilyreverse direction to reverse bias the LED and cause a relatively largeramount of the current pulse to flow through the thermistor and arelatively smaller amount of the current pulse to flow through the LEDas a leakage current; and determining a temperature of the thermistorbased on a portion of the current pulse flowing therethrough.
 15. Themethod according to claim 14 including using the thermistor temperaturein a calibration procedure to increase an accuracy of the determinationof the operating temperature of the LED.
 16. The method according toclaim 1 further comprising: receiving with a ballast a utility powerfrom a utility power source, the utility power in the form of a utilityalternating current and a utility alternating voltage, the alternatingvoltage having a root-mean-square voltage over 100 Volts, and whereinproviding the operating current to the LED includes converting theutility alternating current into the operating current using theballast; and configuring the ballast to a size that is smaller than avolume envelope of a conventional single gang junction box.
 17. Themethod according to claim 1 further comprising: determining multipleoperating temperatures at multiple different times and saving values ofthe temperatures and times at which the temperatures were determined foruse in determining an overall operable lifetime of the LED.
 18. Themethod according to claim 17 further comprising: determining the overalloperable lifetime of the LED based at least in part on the multipleoperating temperatures and the multiple times at temperature.
 19. Themethod according to claim 18 further comprising: indicating the overalloperable lifetime of the LED to a user.
 20. The method according toclaim 18 further comprising: providing the determined overall operablelifetime of the LED to a system that monitors power usage.
 21. Themethod according to claim 18, wherein the operation lifetime increaseswhen the LED is operated with the operating temperature below a maximumsafe operating temperature, the method further comprising: comparing theoperating temperature with the maximum temperature; and reducing theoperating current to reduce the operating temperature below the maximumoperating temperature to increase the operational lifetime of the LED.22. The method according to claim 1 wherein there are at least two LEDsin the circuit which are both electrically connected to the first andsecond LED conductors and wherein the superimposed current pulseincludes a frequency component at a predetermined frequency, the methodfurther comprising: passing the current pulse to a first one of the LEDsbased on the frequency component while blocking the current pulse to thesecond one of the LEDs based on the frequency component and wherein thedetermined operating temperature is the operating temperature of thefirst LED.
 23. The method according to claim 1 wherein the determinedoperating temperature is a first operating temperature which occurs at afirst time and further comprising determining a second, differentoperating temperature at a second, different time.
 24. The methodaccording to claim 1 wherein the provided operating voltage is between1.5 Volts and 2.5 Volts DC.
 25. The method according to claim 1 whereinthe superimposed current pulse is a current ramp having a plurality ofdifferent current magnitudes and the sensed voltage pulse is a voltageramp having a plurality of different voltage magnitudes corresponding tothe current magnitudes such that the operating temperature is determinedbased on a curve created by using a log of a plurality of the currentsplotted against the corresponding plurality of voltages.
 26. The methodaccording to claim 1 wherein superimposing the current pulse, sensingthe voltage pulse, determining the current magnitude and determining theoperating temperature are performed periodically.
 27. A method fordetermining a temperature near at least one light emitting diode (LED)in a circuit which includes a power supply for powering the LED throughfirst and second LED conductors to cause the LED to operate to generatelight when a forward operating voltage and forward operating current issupplied to the LED through the LED conductors, the LED exhibits aforward voltage resistance when the forward operating voltage issupplied to the LED through the LED conductors and the LED exhibits areverse bias voltage resistance when a reverse bias voltage is suppliedto the LED through the LED conductors, where the reverse bias voltageresistance is greater than the forward voltage resistance, the methodcomprising: arranging a thermistor across the first and second LEDconductors in parallel with the LED, the thermistor having an effectiveresistance range in which at least two different thermistor resistancesof the thermistor correspond to at least two different thermistortemperatures of the thermistor, and where the thermistor resistances inthe effective resistance range are lower than the reverse bias voltageresistance of the LED and are higher than the forward voltage resistanceof the LED; selecting the effective resistance range such that, when aforward drive current is applied to the LED conductors, one portion ofthe forward drive current which flows through the LED is the forwardoperating current and another portion of the forward drive current whichflows through the thermistor is a forward thermistor current which issmaller than the forward operating current and when the reverse biasvoltage is supplied to the LED conductors, a reverse drive current flowsthrough the LED conductors in an opposite direction than the forwarddrive current and one portion of the reverse drive current flows as aleakage current through the LED and which does not cause the LED toproduce light and another portion of the reverse drive current flowsthrough the thermistor as a reverse thermistor current which is largerthan the leakage current; and positioning the thermistor in a thermalpathway of the LED to receive heat produced by the LED during operationof the LED, such that the temperature of the thermistor is measurable bydetermining the thermistor resistance using the reverse thermistorcurrent and the temperature of the thermistor is related to atemperature of the LED.
 28. The method according to claim 27 wherein theLED includes a package with a lens and an LED die and positioning thethermistor includes integrating the thermistor in the LED package. 29.The method according to claim 27, further comprising: applying thereverse bias voltage to the LED conductors; measuring the reversethermistor current; calculating the thermistor resistance based on thereverse bias voltage and reverse thermistor current; and determining thetemperature of the thermistor based on the calculated thermistorresistance.
 30. The method according to claim 29, further comprising:applying a forward bias voltage to the LED conductors after theapplication of the reverse bias voltage to the LED conductors to causethe LED to produce light.
 31. The method according to claim 29, whereinthe applied reverse bias voltage is smaller in magnitude than a reversebias breakdown voltage of the LED.
 32. The method according to claim 29further comprising using the determined temperature of the thermistor asa calibration value in a technique that monitors the LED temperaturebased on current flow in the LED conductors and voltage across theconductors.
 33. The method according to claim 29 wherein the temperatureof the thermistor is determined while the LED is at an ambienttemperature.
 34. A switch assembly for electrical communication with atleast one light emitting diode (LED) assembly to allow the switchassembly to control the LED assembly, the LED assembly having at leastone LED with a first LED conductor and a second LED conductor and whichis powered by receiving an operating current through the LED conductorsin a range of operating currents and an operating voltage across the LEDconductors in a range of operating voltages, where the LED assembly isconfigured to be fixedly installed remotely from the switch assembly toprovide light, the switch assembly comprising: a power supply forelectrically connecting to the first and second LED conductors forproviding the operating current in the range of operating currents andthe operating voltage in the range of operating voltages; a temperaturemonitor for electrically connecting to the LED conductors, thetemperature monitor including a current sensor to determine a magnitudeof current through the LED conductors and including a voltage sensor todetermine a magnitude of voltage across the LED conductors, thetemperature monitor also including a controller electrically connectedwith the current and voltage sensors and configured for calculating atemperature of the LED in the LED assembly based at least partially onthe determined current and voltage magnitudes; and a switch forselectively connecting and disconnecting the operating current andoperating voltage from the LED assembly.
 35. A switch assembly asdefined in claim 34, wherein the switch assembly is arranged to fitwithin a volume envelope of a conventional single gang junction box. 36.A switch assembly as defined in claim 34, wherein the switch furtherincludes a dimming controller arranged to selectively reduce theoperating current to the LED assembly to reduce light produced by theLED assembly.
 37. A switch assembly as defined in claim 34 wherein thetemperature monitor is arranged to control the power controller tosuperimpose a current pulse on the operating current to the LED assemblythrough the first and second LED conductors resulting in a voltage pulsethat is superimposed on the operating voltage, and wherein thedetermined magnitude of current is the magnitude of the current pulseand the determined magnitude of voltage is the magnitude of the voltagepulse and the controller calculates the temperature of the LED based atleast partially on the magnitude of the current pulse and the magnitudeof the voltage pulse.
 38. A switch assembly as defined in claim 37,wherein the LED is operable at a safe operating temperature that is lessthan a maximum temperature and is subject to thermal damage at a damagetemperature that exceeds the maximum temperature, and wherein thetemperature monitor is arranged to compare the determined temperature ofthe LED to the maximum temperature and to control the power controllerto reduce the operating current if the determined temperature exceedsthe maximum temperature.
 39. A switch assembly as defined in claim 37,wherein the superimposed current pulse is a first current pulse and thevoltage pulse is a first voltage pulse, and wherein the temperaturemonitor is further arranged to control the power controller tosuperimpose at least a second, different current pulse on the operatingcurrent which results in a second, different voltage pulse that issuperimposed on the operating voltage, the temperature monitor alsoarranged to derive an operating temperature curve from a logarithm of amagnitude of the current pulses against a magnitude of the voltagepulses and for determining a slope of the operating temperature curveand using the determined slope in calculating the temperature of theLED.
 40. A switch assembly as defined in claim 34, further comprising aclock for monitoring an overall time that the LED is operating and amemory for storing the determined temperature and time at the determinedtemperature and wherein the switch assembly further comprises a displaythat is configured at least to display information about a lifetime ofthe LED based at least in part on the determined temperature, the timeat the determined temperature and the overall time that the LED isoperating.
 41. A switch assembly as defined in claim 34, furthercomprising a clock for monitoring an overall time that the LED isoperating and wherein the switch assembly further comprises a displaythat is configured at least to display information about a powerconsumption of the LED based at least on the overall time that the LEDis operating and the operating current.
 42. A switch assembly as definedin claim 34 wherein the switch assembly further comprises a display thatis configured at least to display information about the temperature ofthe LED.
 43. A switch assembly as defined in claim 34 wherein the firstand second LED conductors are a first set of LED conductors and theaforesaid LED assembly is a first LED assembly and wherein thetemperature monitor is arranged to electrically connect with at least asecond LED assembly through a second set of LED conductors.
 44. A switchassembly as defined in claim 43 wherein the temperature monitor isarranged to electrically connect to the first and second sets of LEDconductors when the first and second sets of LED conductors areelectrically connected in parallel.
 45. A switch assembly as defined inclaim 43 wherein the temperature monitor is arranged to electricallyconnect to the first and second sets of LED conductors when the firstand second sets of LED conductors are electrically connected in series.46. A switch assembly as defined in claim 34 wherein the switch includesa data acquisition device for gathering information about the LED andthe switch assembly further comprises a wireless system for transferringthe gathered information to a system that is external to the switch. 47.A switch assembly as defined in claim 46 wherein the external systemmonitors power usage in a building.
 48. A switch assembly as defined inclaim 34 wherein the switch communicates to a system that is external tothe switch using Ethernet.
 49. A switch assembly as defined in claim 34wherein the switch communicates to a building information system that isexternal to the switch using a wireless interface.
 50. A switch assemblyas defined in claim 34 wherein the temperature monitor is arranged tocontrol the power controller to superimpose a voltage pulse on theoperating voltage to the LED assembly across the first and second LEDconductors resulting in a current pulse that is superimposed on theoperating current, and wherein the determined magnitude of voltage isthe magnitude of the voltage pulse and the determined magnitude ofcurrent is the magnitude of the current pulse and the controllercalculates the temperature of the LED based at least partially on themagnitude of the voltage pulse and the magnitude of the current pulse.51. A switch assembly as defined in claim 34 wherein the power supplyincludes a transformer for electrically connecting to a line powersource for receiving line power with a line voltage greater than 100Volts AC and converting the line voltage to a transformed power with atransformed voltage that is less than 50 Volts; and the power supplyalso includes a power controller for receiving the transformed powerfrom the transformer and for at least creating an operating power withthe operating current in the range of operating currents and theoperating voltage in the range of operating voltages, the powercontroller arranged to electrically connect to the LED assembly throughthe LED conductors to supply the operating current and operating voltageto the LED assembly through the LED conductors;
 52. A control system fordetermining a temperature of at least one light emitting diode (LED)having first and second LED conductors and where the LED is operable togenerate light in response to receiving an operating current in a rangeof operating currents and receiving an operating voltage in a range ofoperating voltages, and the LED is operable at an operating temperature,the control system comprising: a power supply for electricallyconnecting to the first and second LED conductors and for creating theoperating current and operating voltage and for providing the operatingcurrent and operating voltage to the LED through the first and secondLED conductors, and the power supply configured for use in superimposingat least one current pulse on the operating current to the LED, throughthe first and second LED conductors resulting in a voltage pulse that issuperimposed on the operating voltage; a voltage sensor for sensing thevoltage pulse across the first and second LED conductors resulting fromthe applied pulse of current to determine a voltage magnitude of thevoltage pulse; a current sensor for determining a current magnitude ofthe current pulse; and a controller for determining the operatingtemperature of the LED at the provided operating current based on thecurrent magnitude of the current pulse and the voltage magnitude of thevoltage pulse.
 53. The control system according to claim 52, wherein thecontroller is configured to control the power supply to provide aninitial operating current and initial operating voltage at a knowninitial temperature of the LED before providing the operating current,and the controller uses the measured values of the initial operatingcurrent and initial operating voltage and initial temperature todetermine at least one LED parameter of the LED, and the controller isconfigured to determine the operating temperature using the determinedLED parameter.
 54. The control system according to claim 52 wherein thecontroller is configured to control the power supply to provide theinitial operating current and initial operating voltage at a time whenthe LED has reached a resting temperature that is essentially the sameas an ambient temperature around the LED.
 55. The control systemaccording to claim 52 wherein the controller is configured to determinethe operating temperature using the LED parameter in an ideal diodeequation.
 56. The control system according to claim 55 wherein the LEDparameter is related to an effective band gap of the LED.
 57. Thecontrol system according to claim 55 wherein the LED parameter is usedto determine a current to voltage curve for at least one temperature.58. The control system according to claim 52 wherein the superimposedcurrent pulse is a first current pulse and the voltage pulse is a firstvoltage pulse, and wherein the controller is configured to control thepower supply to superimpose at least a second, different current pulseon the operating current which results in a second, different voltagepulse that is superimposed on the operating voltage, and wherein thecontroller is further configured to derive an operating temperaturecurve from a logarithm of a magnitude of the current pulses against amagnitude of the voltage pulses and to determine a slope of theoperating temperature curve, and to determine the operating temperatureof the LED based on the determined slope of the operating temperaturecurve.
 59. The control system according to claim 58 wherein thecontroller determines the operating temperature by comparing the slopeof the operating temperature curve to predetermined slope values thatcorrespond to different LED temperatures.
 60. The control systemaccording to claim 52 wherein the power supply superimposes the currentpulse on the operating current at a predetermined magnitude such thatthe determined current magnitude is the predetermined magnitude.
 61. Thecontrol system according to claim 52 wherein the power supply exhibits aripple current that is used as the current pulse.
 62. The control systemaccording to claim 52 wherein the circuit including the LED and thepower supply is part of a device to at least provide illumination toassist visual perception of humans, and where the power supplysuperimposes the current pulse on the operating current to produces atemporary change in the light level from the LED and the temporarychange in the light level is less than an interval having a durationthat is imperceptible to humans.
 63. The control system according toclaim 52, wherein the LED is operable at a safe operating temperaturethat is less than a maximum temperature and is subject to thermal damageat a damage temperature that exceeds the maximum temperature, andwherein the controller compares the operating temperature with themaximum temperature and reduces the operating current responsive to theoperating temperature exceeding the maximum temperature.
 64. The controlsystem according to claim 52 wherein the power supply superimposes thecurrent pulse in a way which lowers the magnitude of the operatingcurrent to a value that is less than another value of the operatingcurrent that would otherwise be present without the superimposed currentpulse.
 65. The control system according to claim 52 further comprising:a thermistor arranged across the first and second LED conductors inparallel with the LED such that the current pulse flows through aparallel combination of the LED and the thermistor, and wherein thethermistor has an effective resistance range that is lower than areverse bias resistance of the LED and that is higher than a forwardvoltage resistance of the LED, and wherein the power supply superimposesthe current pulse in a way which causes the current to the LED totemporarily reverse direction to reverse bias the LED and which causes arelatively larger amount of the current pulse to flow through thethermistor and a relatively smaller amount of the current pulse to flowthrough the LED as a leakage current, and the controller is configuredto determine a temperature of the thermistor based on a portion of thecurrent pulse flowing therethrough.
 66. The control system according toclaim 65, wherein the controller is configured for using the thermistortemperature in a calibration procedure to increase an accuracy of thedetermination of the operating temperature of the LED.
 67. The controlsystem according to claim 52 further comprising: a ballast for receivinga utility power from a utility power source, the utility power in theform of a utility alternating current and a utility alternating voltage,the alternating voltage having a root-mean-square voltage over 100Volts, and wherein the ballast converts the utility alternating currentinto the operating current and provides the operating current to the LEDand control system, including the ballast, is configured to a size thatis smaller than a volume envelope of a conventional single gang junctionbox.
 68. The control system according to claim 52 wherein the controlleris configured to determine multiple operating temperatures at multipledifferent times, and further comprising: a clock for use in determiningtimes; and a memory for saving values of the temperatures and times atwhich the temperatures were determined for use in determining an overalloperable lifetime of the LED.
 69. The control system according to claim68 further comprising: configuring the controller to determine theoverall operable lifetime of the LED based at least in part on themultiple operating temperatures and the multiple times at temperature.70. The control system according to claim 69 further comprising: adisplay for indicating the overall operable lifetime of the LED to auser.
 71. The control system according to claim 68 further comprising:an interface for providing the determined overall operable lifetime ofthe LED to a separate, different system that monitors power usage. 72.The control system according to claim 68, wherein the overall operablelifetime increases when the LED is operated with the operatingtemperature below a maximum safe operating temperature, the controllerfurther configured to compare the operating temperature with the maximumtemperature and reduce the operating current to reduce the operatingtemperature below the maximum operating temperature to increase theoperational lifetime of the LED.
 73. The control system according toclaim 50 wherein there are at least two LEDs in the circuit which areboth electrically connected to the first and second LED conductors, andwherein the power supply superimposes the current pulse with a frequencycomponent at a predetermined frequency, further comprising: a firstfrequency filter positioned for passing the current pulse to a first oneof the LEDs based on the frequency component; and a second frequencyfilter positioned for blocking the current pulse to the second one ofthe LEDs based on the frequency component, and wherein the controllerdetermines the operating temperature of the first LED.
 74. The controlsystem according to claim 50 wherein the operating temperaturedetermined by the controller is a first operating temperature whichoccurs at a first time and further comprising configuring the controllerto determine a second, different operating temperature at a second,different time.
 75. The control system according to claim 50 wherein thepower supply is arranged to provide the operating voltage at 1.5 Voltsto 2.5 Volts DC.
 76. The control system according to claim 50 whereinthe power supply superimposes the current pulse in the form of a currentramp having a plurality of different current magnitudes and the sensedvoltage pulse is a voltage ramp having a plurality of different voltagemagnitudes corresponding to the current magnitudes, and the controlleris arranged to determine the operating temperature based on a curvecreated by using a log of a plurality of the currents plotted againstthe corresponding plurality of voltages.
 77. The control systemaccording to claim 50 wherein the control system is arranged tosuperimpose the current pulse, sense the voltage pulse, determine thecurrent magnitude and determine the operating temperature periodically.78. The control system according to claim 50 wherein at least a portionof the control system is integrated into a chip.
 79. A control systemfor determining a temperature near at least one light emitting diode(LED) in a circuit which includes a power supply for powering the LEDthrough first and second LED conductors to cause the LED to operate togenerate light when a forward operating voltage and forward operatingcurrent is supplied to the LED through the LED conductors, the LEDexhibits a forward voltage resistance when the forward operating voltageis supplied to the LED through the LED conductors and the LED exhibits areverse bias voltage resistance when a reverse bias voltage is suppliedto the LED through the LED conductors, where the reverse bias voltageresistance is greater than the forward voltage resistance, the methodcomprising: a thermistor electrically connected across the first andsecond LED conductors in parallel with the LED, the thermistor having aneffective resistance range in which at least two different thermistorresistances of the thermistor correspond to at least two differentthermistor temperatures of the thermistor, and where the thermistorresistances in the effective resistance range are lower than the reversebias voltage resistance of the LED and are higher than the forwardvoltage resistance of the LED, and where the effective resistance rangeis such that, when a forward drive current is applied to the LEDconductors, one portion of the forward drive current which flows throughthe LED is the forward operating current and another portion of theforward drive current which flows through the thermistor is a forwardthermistor current which is smaller than the forward operating currentand when the reverse bias voltage is supplied to the LED conductors, areverse drive current flows through the LED conductors in an oppositedirection than the forward drive current and one portion of the reversedrive current flows as a leakage current through the LED and which doesnot cause the LED to produce light and another portion of the reversedrive current flows through the thermistor as a reverse thermistorcurrent which is larger than the leakage current; and wherein thethermistor is positioning in a thermal pathway of the LED to receiveheat produced by the LED during operation of the LED, such that thetemperature of the thermistor is measurable by determining the reversethermistor current and the temperature of the thermistor is related to atemperature of the LED.
 80. The control system according to claim 79,wherein the LED includes a package with a lens and an LED die and thethermistor is integrated in the LED package.
 81. The control systemaccording to claim 79, further comprising: a power supply electricallyconnected to the LED to supply power to the LED; a controllerelectrically connected to the power supply to control the power supplyto apply a reverse bias voltage to the LED conductors; a sensorconnected to at least one of the LED conductors to measure the reversebias current produced by applying the reverse bias voltage to the LEDconductors, and connected with the controller; and where the controlleris arranged to calculate the thermistor resistance based on the reversebias voltage and reverse thermistor current and to determine thetemperature of the thermistor based on the calculated thermistorresistance.
 82. The control system according to claim 81, wherein thecontroller is also arranged to control the power supply to apply aforward bias voltage to the LED conductors after the application of thereverse bias voltage to the LED conductors to cause the LED to producelight.
 83. The control system according to claim 81, wherein thecontroller controls the power supply to apply the reverse bias voltageat a magnitude that is smaller than a reverse bias breakdown voltage ofthe LED.
 84. The control system according to claim 81 wherein thecontroller is further configured to use the determined temperature ofthe thermistor as a calibration value in a technique that monitors theLED temperature based on current flow in the LED conductors and voltageacross the conductors.
 85. The control system according to claim 81wherein controller determines the temperature of the thermistor whilethe LED is at an ambient temperature.
 86. A method for electricallycommunicating with at least one light emitting diode (LED) assembly tocontrol the LED assembly, the LED assembly having at least one LED witha first LED conductor and a second LED conductor and which is powered byreceiving an operating current through the LED conductors in a range ofoperating currents and an operating voltage across the LED conductors ina range of operating voltages, where the LED assembly is configured tobe fixedly installed to provide light, the method comprising: providingthe operating current in the range of operating currents and theoperating voltage in the range operating voltages; selectivelyconnecting and disconnecting the operating current and operating voltageto the LED assembly through the LED conductors to control light outputof the LED assembly; and determining a temperature of the LED throughthe LED conductors at least partially by determining a magnitude ofcurrent through the LED conductors and determining a magnitude ofvoltage across the LED conductors.
 87. A method as defined in claim 86,wherein said transforming the line power, receiving the transformedpower, selectively supplying the operating current and operating voltageand determining the temperature are all done within a volume envelope ofa conventional single gang junction box.
 88. A method as defined inclaim 86, wherein selectively supplying the operating current andoperating voltage further includes dimming by selectively reducing theoperating current to the LED assembly to reduce light produced by theLED assembly.
 89. A method as defined in claim 86 wherein thetemperature is determined at least in part by superimposing a currentpulse on the operating current to the LED assembly through the first andsecond LED conductors resulting in a voltage pulse that is superimposedon the operating voltage, and wherein the determined magnitude ofcurrent is the magnitude of the current pulse and the determinedmagnitude of voltage is the magnitude of the voltage pulse and thetemperature of the LED is determined based at least partially on themagnitude of the current pulse and the magnitude of the voltage pulse.90. A method as defined in claim 89, wherein the LED is operable at asafe operating temperature that is less than a maximum temperature andis subject to thermal damage at a damage temperature that exceeds themaximum temperature, and further comprising: comparing the determinedtemperature of the LED to the maximum temperature and reducing theoperating current if the determined temperature exceeds the maximumtemperature.
 91. A method as defined in claim 89, wherein thesuperimposed current pulse is a first current pulse and the voltagepulse is a first voltage pulse, further comprising: superimposing atleast a second, different current pulse on the operating current whichresults in a second, different voltage pulse that is superimposed on theoperating voltage; determining an operating temperature curve from alogarithm of a magnitude of the current pulses against a magnitude ofthe voltage pulses; determining a slope of the operating temperaturecurve; and using the determined slope in determining the temperature ofthe LED.
 92. A method as defined in claim 86, further comprising:monitoring an overall time that the LED is operating; storing thedetermined temperature and time at the determined temperature; anddisplaying information about a lifetime of the LED based at least inpart on the determined temperature, the time at the determinedtemperature and the overall time that the LED is operating.
 93. A methodas defined in claim 86, further comprising: monitoring an overall timethat the LED is operating; and displaying information about a powerconsumption of the LED based at least on the overall time that the LEDis operating and the operating current.
 94. A method as defined in claim86, further comprising: displaying information about the temperature ofthe LED.
 95. A method as defined in claim 86 wherein the first andsecond LED conductors are a first set of LED conductors and theaforesaid LED assembly is a first LED assembly, further comprising:selectively supplying the operating current and operating voltage to atleast a second LED assembly having a second LED through a second set ofLED conductors and determining a temperature of the second LED throughthe second set of LED conductors.
 96. A method as defined in claim 95wherein the operating current and operating voltage are selectivelysupplied to the first and second sets of LED conductors when the firstand second sets of LED conductors are electrically connected inparallel.
 97. A method as defined in claim 95 wherein the operatingcurrent and operating voltage are selectively supplied to the first andsecond sets of LED conductors when the first and second sets ofconductors are electrically connected in series.
 98. A method as definedin claim 86 further comprising: gathering information about the LED; andwirelessly transmitting the gathered information to a separate,different system.
 99. A method as defined in claim 98 wherein theseparate, different system monitors power usage in a building andwherein said gathering information includes gathering information aboutpower usage of the LED.
 100. A method as defined in claim 86 furthercomprising: gathering information about the LED; and transmitting thegathered information to a separate, different system using Ethernet.101. A method as defined in claim 86 further comprising: gatheringinformation about the LED; and transmitting the gathered information toa separate, different building information system using a wirelessinterface.
 102. A method as defined in claim 86 wherein the temperatureis determined at least in part by superimposing a voltage pulse on theoperating voltage to the LED assembly across the first and second LEDconductors resulting in a current pulse that is superimposed on theoperating current, and wherein the determined magnitude of voltage isthe magnitude of the voltage pulse and the determined magnitude ofcurrent is the magnitude of the current pulse and the controllercalculates the temperature of the LED based at least partially on themagnitude of the voltage pulse and the magnitude of the current pulse.103. A method as defined in claim 86, wherein providing the operatingcurrent and operating voltage includes: transforming a line power sourcewith a line voltage greater than 100 Volts AC and converting the linevoltage to a transformed power with a transformed voltage that is lessthan 50 Volts; and receiving the transformed power and at least creatingan operating power with the operating current in the range of operatingcurrents and the operating voltage in the range of operating voltagesfrom the transformed power.